1. Field of the Invention
The present invention relates generally to circuit boards, and more particularly to multilayer circuit boards having air bridge crossover circuits.
2. Disclosure Information
In the field of electronics manufacturing, various additive and subtractive processes are known for constructing printed circuit boards (PCBs). Among these are the processes disclosed in U.S. Pat. Nos. 4,404,059 to Livshits et al. (hereinafter"Livshits"), U.S. Pat. No. 3,801,388 to Akiyama et al. (hereinafter"Akiyama"), and U.S. Pat. No. 5,738,797 to Belke et al. (hereinafter "Belke"), all three of which are incorporated herein by reference. These sources describe various additive and subtractive plating and chemical etching processes for constructing multilayer (i.e., two-or-more-layer) circuit boards having air bridges.
As typified in FIGS. 1A and 2A-F (and in FIGS. 1A-D of Akiyama), the subtractive process involves the following steps. First (FIG. 2A), a tri-metal-layer laminate is provided, comprising a central layer 10 made of a first metal, and upper and lower conductor network layers 32/34 made of a second metal. This tri-metal-laminate may be produced by cladding the second-metal upper and lower layers onto the first-metal central layer, or by panel-plating the second metal onto top and bottom surfaces of a first-metal foil, or the like. Second (FIG. 2B), upper and lower etch-resist masks 22/24 are disposed on the exposed surfaces of the upper and lower conductor network layers 32/34. The upper and lower masks 22/24 correspond to predefined upper and lower circuit patterns to be formed in the respective conductor network layers 32/34. The masks 22/24 also have respective apertures 23/25 therein through which portions of the conductor network layers are exposed. third (FIG. 2C), the masked laminate is exposed to an etchant which etches substantially only the second metal (without appreciably etching the first metal), such that the portions of the second metal exposed through the mask apertures are etched away. Fourth (FIG. 2D), the masks are stripped off, fifth (FIG. 2E), the structure is attached to an electrically insulative surface 61 of a substrate 60, and sixth (FIG. 2F) the structure is exposed to an etchant which etches substantially only the first metal, so as to etch away substantially all of the metal foil except for those portions 16 sandwiched between the top and bottom conductor networks. These sandwiched portions 16 are much less aggressively etched because the top and bottom conductor networks act essentially as etch-resist masks, since the metal of these networks is not appreciably etched by the aforementioned etchant. After etching, these sandwiched portions 16 typically assume an hour-glass-like shape, and are referred to as "pedestals", "support elements", or the like.
An advantage that air bridge crossover circuits provide over conventional FR-4 PCBs is that they facilitate high density circuits--that is, they permit the arrangement of a high number of circuit traces and other circuit elements into a small circuit board area. However, such circuits are often susceptible to mechanical damage (e.g., from impact, thermal excursions, and the like) and/or to poor electrical performance (e.g., caused by the use of certain metals having poor electrical conductance characteristics as the conductor network material). Thus, it would be desirable to provide a way of producing such multilayer air bridge circuits without these drawbacks.